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1H and 13C NMR study of catalytic reaction between CH3OH and H2S
Jouml
of Molecular Catalysis, 66 (1991) 73-83
73
‘H and 13C NMR study of catalytic reaction between CH30H and H2S A. V. NOSOV,V. M. Mastikhin and A. V. Mashkina Institute of Catalysis, Siberian Branch of the Acadmny of Sc&nces, Novosibirsk, 630090 (-US.S.R.] (Received May 30, 1990; revised December 5, 1990)
Abstract The resultsof ‘H and 13C NMR as well as kinetic studies of the reaction between CH30H and HeS over a number of solid catalvsts (SiOe, H3P0&i02, NaX, NaY and H-ZSM-5 zeolites) are presented. The adsorbed methanol forms methoxy groups on H3P0JSi02 and presumably on H-ZSM-5 catalysts while for SiOe, NaX and NaY only physically adsorbed methanol molecules are found on the catalyst surface. The ‘H and 13C NMR spectra in combination with the kinetic data indicate that the adsorbed.CH,OH reacts with SH- species and HzS molecules, the former being more reactive.
Introduction Zeolites as well as oxides are known to catalyze the reaction between CHaOH and H2S, with formation of methylmercaptan (MM) and dimethyl sulphide (DMS). The mechanism of this reaction has not been elucidated. Ziolek et al. [ 1, 21 studied this reaction over HNaY zeolite by means of IR spectroscopy. They obtained some evidence that the reaction proceeds via methoxylation of the zeolite surface, with subsequent reaction between methoxy groups and physically adsorbed H2S. Both MM and DMS are formed at low temperatures; at higher temperatures DMS formation prevails. In [3] Mashkina et al. have analyzed the effect of acid-base properties of a wide range of catalysts on their activity in the reaction of methanol with hydrogen disulphide. They concluded that on catalysts having strong acidic sites the reaction proceeds fust between methoxy groups and dissociatively adsorbed Has, with formation first of MM which transforms into DMS. Recently NMR spectroscopy has become a powerful tool for the study of surface active sites, adsorbed molecules and their reactions over catalyst surface [ 4,5]. Derouane et al. [6] were the first to apply 13CNMR spectroscopy to the study of methanol conversion over ZSM-5 zeolites. The use of magic angle spinning (MAS) results in the high resolution NMR spectra of adsorbed molecules and thus provides more detailed information on the chemical nature of the absorbed species. In this paper we present data on the interaction of CHsOH with H$ over a number of solid catalysts obtained via ‘H and 13C NMR (including 0304-5102/91/$3.50
Q Elsevier Sequoia/Printed in The Netherlands
74
MAS experiments), as well as by kinetic studies of this reaction. The simultaneous measurement of 13C and ‘H NMR spectra have provided us with information on the products of this reaction and allowed us to make some conclusions regarding the reaction mechanism.
Experimental NaX, NaY and H-ZSM-5 zeolites were commercial samples purchased from Gorky Chemical Plant. The framework Si/Al ratios were 2.5, 4.7 and 60 respectively. Commercial SiOz (KSK type, specific area 280 m2 g-‘) was used. The samples were calcined in air at 400 “0 (NaX and NaY) and 500 “C (H-ZSM-5 and Si02) for 2 h. After H3P04 impregnation from aqueous solution, the Si02 samples were dried in air at 110 “C for 5 h, with subsequent calcination in He flow at 400 “C for 2 h. The continuous flow technique, with gas chromatographic analysis of the reaction products, was used for measurements of catalytic activity [3]. Experiments were carried out in the kinetic region on catalyst granules 0.25-0.5 mm diameter at 300-360 “C, P=O.l MPa and H2S:CH30H ratio @I) 0.6 and 1.6 without dilution of reactants at different contact times T, (s). The catalytic activity was characterized by methanol conversion rate W (mm01 g- ’ h-l). The selectivity was determined as a ratio of the product yield to methanol conversion (%). NMR spectra were measured at 300.066 (‘H) and 75.43 MHz (13C) using a Bruker CXP-300 NMR spectrometer at ambient temperature. Prior to the measurements the samples were placed in glass tubes of two types: ‘MAS’ (7 mm o.d. and 12 mm length) for measurements of ‘H and 13C MAS NMR spectra or ‘HR’ (10 mm o.d. and 200 mm length) for measurements of 13C spectra Without sample spinning. Samples were evacuated at 500 ‘C (H3P04/ Si02 - at 400 “C) and P= 10m3 Pa for 6 h. Methanol (30% enriched by 13C isotope) adsorption was carried out from alcohol vapour at room temperature. After CH30H adsorption the samples in ‘HR’ tubes were sealed off from the vacuum line and their 13C spectra were recorded. Then the ‘HR tubes were again sealed to the vacuum line and H2S adsorption was carried out. Samples in ‘MAS’ tubes were sealed off immediately after H2S adsorption and CH30H subsequent co-adsorption; only H2S was adsorbed on some samples. The quantities of adsorbed substances were determined volumetrically. After H2S and CH30H adsorption the samples were heated at temperatures from 100 to 400 “C for 20 min outside the NMR spectrometer, with subsequent measurements of the spectra. The chemical shifts were measured relative to external TMS. The rotation frequency of the sealed samples was 2.7-3 KHz.
75
Results
‘H and 13CNMR spectra CH,OH adswption 13CNMR spectra of CH30H adsorbed on catalysts axe shown in Fig. 1. The spectrum of CH30H/Si02 exhibits a line at 6=52.0 ppm with a width of 300 Hz. (Fig. l(a)). The 13C chemical shift of liquid CH30H is 49.4 ppm [ 71. Small differences in chemical shifts between liquid CH,OH and that
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Fig. 1. I% NMR spectra of CHBOH adsorbed on (a) SiOz; (b) H3POJSi02; (c) H-ZSM-5; (d) Nay. Fig. 2. 13CNMR spectra of CH,OH adsorbed on H3P0,/Si02 after Hz3 coadsorption and heating at (a) 100 “C; (b) 200 “C; (c) 250 “C; (d) 300 “C; (e) 350 “C; (f) 400 “C; M= 1.0.
76
adsorbed on SiOz may be due to the differences in the magnetic susceptibilities of the samples. Methanol adsorption on H3P04/Si02 gives rise to two overlapping lines at 6= 51.4 and 60 ppm (Fig. l(b)) belonging to physically adsorbed CHaOH (6= 51.4 ppm) and to some more rigidly bound species, most probably surface methoxy groups. The line at S = 54 ppm is observed in the spectrum of CH,OH/‘H-ZSM-5 (Fig. I(c)). The spectra of methanol adsorbed on NaX and NaY zeolites are very similar. As an example, the spectrum of CH,OH/NaY with the line at 6= 52.4 ppm from physically adsorbed methanol is shown in Pig. l(d). Reaction of CH,OH with Ha (a) H3p041Si02. H$ adsorption on H3P0,JSi02 with preadsorbed CHBOH and subsequent heating at 100 “C does not change the 13C spectrum (Fig. 2(a)). An increase in temperature gives rise to the narrow line at 61 ppm accompanied by a decrease in the intensity of the line at 52 ppm (Pig. 2(b-d)). After heating at 350 “C the latter vanishes and new lines at 6= 31.4 and 9.1 ppm appear. Heating at 400 “C gives rise to a small signal at S- 16 ppm. Chemical shift of the low-field line only slightly deviates from that of liquid dimethyl ether (DME, 6=59.4 ppm [7]) and most probably belongs to physically adsorbed DME. The remaining signals (at 6= 31.4, 16 and 9.1 ppm) may be attributed to the products of thermal decomposition of DME, most probably branched alkanes. (b) H-ZSM-5. After H$ adsorption the line from adsorbed CH30H shifts downfield and broadens compared to the spectrum of CH,OHM-ZSM-5: 6=56.2 and 54.0 ppm, AV,~= 850 and 710 Hz respectively (Pig. 3(a)). Heating the sample at 100 “C gives rise to signals at 29.5 and 6.3 ppm, the low-field line broadens to 940 Hz and becomes slightly asymmetric. The high-field (S=6.3 ppm) signal most probably belongs to MM (liquid MM 6=6.5 ppm [7]). After heating at 160 “C (Pig. 3(a)) the intensity of the signal at low field decreases and the signal at 29.5 ppm increases, while the signal from MM does not change. Heating at 210 “C results in the disappearance of the low-field signal and gives rise to a signal at 6 = 18.7 ppm which we have attributed to DMS (liquid DMS S= 18.5 ppm 171). Heating at 350 “C does not change the spectrum. (c) NuY. H2S adsorption and heating at temperatures from 100 to 200 “C lead to line broadening to 830 Hz (Avln of CH30H adsorbed at room temperature was 200 Hz) (F’ig. 4(a-d)). Signals from DME and MM appear in the spectrum measured after the sample was heated at 250 “C (Fig. 4(e)). Heating at 300 “C gives rise to the line at 6= 18.0 ppm from DMS accompanied by an increase in intensity of the signal from DME (Pig. 4(f)). Heating at higher temperature (400 “C) leads to disappearance of the line from MM (Fig. 4(g)). (d) NuX. Figure 5 shows ‘H and “C NMR MAS spectra obtained for M =0.4. The line widths in 13C MAS spectra are 4 times smaller than those measured without sample spinning. The molecules which do not contain
29.5
8
.
. 100
.
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.
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50
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Fig. 3. 13C NMR spectra of CH30H adsorbed on H-ZSM-5 after HzS coadsorption (a) and heating at (b) 100 ‘T; (c) 160 “C; (d) 210 “C; (e) 360 “C; M=0.6.
Fig. 4. 13C NMR spectra of CH30H adsorbed on NaY after HzS adsorption (a) and heating at (II) 100 “C; (c) 150 T; (d) 200 “C; (e) 250 “C; Q 300 “C; (g) 400 “C; M=0.4.
carbon atoms, such as H,S (6= 1.0 ppm) and SH- species (S- - 3.6 ppm) [8] along with other compounds ‘visible’ in 13Cspectra, ‘were observed in the ‘H MAS spectra (Fig. 5(a-f)). Heating at 225 “C gives rise to signals from MM, DMS and DME (Fig. 5(c’)). After heating at 250 “C the concentration of DMS on the catalyst surface increases and those of MM and CH30H decrease. The line from MM vanishes after heating at 300 “C and the intensity of the DMS line increases, while the line from DME does not change. After heating at 400 “C (F’ig. 5(f)) the line from CH30H almost disappears, while the intensities of the lines from DMS and DME increase. In the ‘H spectra, the line at 5=4.5 ppm belongs to OH groups of methanol, the line at 2 ppm is due to CH3
78
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so
5
0
25
Od,ppm
-5 6,ppm
Fig. 5. ‘H (a-f) and 13C @‘-f’) NMR MAS spectra of NaX zeolite after (a) H&Sadsorption (5 X 10” molecules g-l); (b, b’) subsequent CHBOH adsorption (1.2 x 102’ g-‘) and heating at (c, c’) 225 “C; (d, d’) 250 “C; (e, e’) 300 “C; (f, f’) 400 “C. Spinning sidebands are marked with asterisks.
protons of DMS and MM, the signal at 3 ppm belongs to CH, protons of methanol and DME. It is well known that on zeolites the dissociative adsorption of HzS occurs with formation of SH- species 191. Recently this result was confirmed by ‘H NMR MAS experiments [8]. According to these data the line at S= 1 .O ppm belongs to the molecular form of adsorbed Has, while the line at S= - 3.6 ppm was attributed to surface SH- species. ‘H spectra show that H2S concentration diminishes as CHaOH conversion increases. It should be mentioned that the concentration of SH- species remains constant while HzS is present on the catalyst surface. The signal from SH- species vanishes only when the signal from adsorbed H2S molecules cannot be seen in the ‘H spectra (Fig. 5(f)). The signal from water formed in the reaction is not observed in the ‘H spectra due to a large line width of about 1 KHz
79
[ lo], which cannot be narrowed upon spinning due to an unfavorable exchange rate with protons of surface OH groups. (e) Nay. Figure 6 shows spectra for M= 1. In contrast to NaX zeolite, the concentration of SH- species is lower due to the smaller Si/Al ratio for this zeolite [9, 81. The signal from methanol OH groups is broadened beyond observation due to the restricted mobility of CH,OH molecules at low surface coverage. Heating of this sample leads to a decrease in the intensity of lines from H2S, CH30H and SH- species, MM being the main product, and quantities of DME and DMS being small. (f) DME +H&3 reaction over Na2i zeolite. DME was obtained in situ by heating adsorbed CHaOH at 400 “C for 30 min (Fig. 7(a’)). All methanol was reacted after this treatment, as can be seen from the 13C spectrum. After contact of the sample with HaS and subsequent heating, the lines from MM and DMS appear in the 13C spectra, accompanied by a corresponding
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6. ‘H (a-e) and 13C @‘-e’) NMR MAS spectra of NaY zeolite after (a) H2S adsorption (2 x 1Olg g-l); (b, b’) subsequent CH30H adsorption (2 x 1020 g-l) and heating at (c, c’) 100 “C; (d, d’) 150 “C; (e, e’) 200 “C.
Fig.
80
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Fig. 7. ‘H (a-f) and 13C (a’-f’) NMR MA3 spectra of NaX zeolite after (a, a’) DME (1 X 102’ g-l) and subsequent H&Sadsorption (2 x 102’ g-l) and heating at (b, b’) 150 “C; (c, c’) 250 “C; (d, d’) 300 “C; (e, e’) 350 “C; (f, f’) 400 “C.
decrease in intensity of the DME line. DMS is the final product of this reaction. SH- species appeared only after heating at 150 “C (Fig. 7(b)) and disappeared only after H$ had vanished, as in the case of CH,OH+ H2S reaction. Catalytic experiments In the presence of all catalysts tested in the H@+CH,OH
reaction at different T, MM, DMS and DME were formed. On increasing the contact time (the reaction temperature and M being constant), conversion X increased. Table 1 shows the results of these experiments. (a) H&‘04/Si02. At M= 0.6 and 1.6, r=9-42 s and 360 “C (as well as at lower temperatures), DME is the main product; at larger r MM and DMS were also formed. (b) H-ZSM-5. At M=0.6 and 1.6 and ~=0.1-1.3 s, DMS was the main product; only small quantities of MM were formed and no DME was detected. DME was observed at 260-300 “C and low methanol conversions.
81 TABLE 1 Conversion rates and selectitities of catalysts in H,S + CH,OH reaction at 360 “C, X= 80% Catalyst
SiOz H3P0,/Si02 H-ZSM-5 NaY NaX
S” cm2 g-l)
360 90 500 800 800
M=0.6
M= 1.6
Tb (9)
WC
85 45 0.9 45 7.0
5 2 3 1 214 6 3.5 58 23 56
Selectivity MMd
“Specifk surface area. bContact time. ‘Methanol conversion rate, mm01 g-’ dMethylmercaptan. eDimethyl sulphide. ‘Dimethyl ether.
W
DMS’
DME’
6 10 91 42 44
92 89 0 0 0
$1
34 20 0.3 7.5 2.5
2 6 250 12 37
Selectivity MM
DMS
DME
25 3 19 72 83
6 30 81 28 17
69 87 0 0 0
h-l.
(c) NaY. At M=0.6 and 1.6 and T= 3-30 s, methanol was converted to MM and DMS; no DME was observed. The selectivity towards MM decreased at increased conversion. (d) NaX. The reaction rates on this zeolite were 7-9 times larger compared with those on NaY, selectivity being almost the same.
Discussion Figure 1 shows that methanol bonding with the surface depends upon the catalyst type. On SiOa, NaX and NaY zeolites, physically adsorbed methanol predominates at coverages used in the experiment (0=0.1-l monolayer). This conclusion follows from the proximity of 13C chemical shifts of liquid, gaseous and adsorbed CH,OH, and from observation of the line from the OH group of methanol adsorbed on NaX zeolite in the ‘H spectra. After CH30H adsorption on H3P04/Si02 two lines were observed, one of them only slightly different from the line of CH30H adsorbed on SiOa. Another line with larger line width and with a chemical shift of N 60 ppm most probably. belongs to Me- O-P groups. The signal of CH,OH adsorbed on H-ZSM-5 zeolite was shifted and broadened compared to methanol adsorbed on SiOz. This may be due to fast exchange between physically adsorbed CH30H and surface methoxy groups. Formation of methoxy groups has been studied by IR spectroscopy on HNaY zeolites [l] as well as by i3C NMR on MgO [ 111. In the latter case, the value of the isotropic chemical shift of methoxy groups was reported to be 60 ppm. The present data indicate that methoxylation occurs on H3P04/
82
SiOz and H-ZSM-5 zeolite. One might expect, however, that the surface of NaY and NaX zeolites, which have small quantities of acidic OH groups, can be also methoxylated to some extent. The formation of a complex between adsorbed CH,OH and H2S seems to take place on NaY zeolite. This follows from the broadening of the methanol 13C signal. No detectable change in chemical shift or appearance of new lines was observed. The line broadening may be explained by decreased mobility of methanol due to formation of its complex with H2S. The bonding energy between HaS and CH,OH molecules in such a complex seems to be rather small because the chemical shift of methanol does not change. The comparison of NMR and gas chromatographic results should be performed with care, because there can be a considerable difference in the data obtained in gas flow and static NMR experiments, as was pointed out recently in [ 121. Keeping this in mind one can see, however, that these two sets of data are in rather good qualitative agreement. Data on reaction over H-ZSM-5 zeolite is the only exception: the product with 5=29.1 ppm is observed in 13C spectra but it is absent in catalytic experiments. This unambiguously indicates that the reaction product having the line with 6 = 29.1 ppm does not desorb from the zeolite surface. Traces of this compound appear also in the reaction over NaY zeolite (Pig. 5(e-f)). Its chemical composition remains unclear, but it should contain sulfur, because no signal with this chemical shift was observed during methanol thermal decomposition on this catalyst. The formation of this product may probably explain the deactivation of this catalyst observed in catalytic experiments. According to literature data, the CH30H + H&l reaction proceeds between methoxy groups and H2S molecules and/or SH- species [l-3]. ‘H NMR is a suitable tool to monitor both HaS and SH- species in this reaction. Concentration of SH- . - *Na+ species on NaX zeolite is greater than that on NaY zeolite, due to a larger concentration of Na+ cations is S(II1) sites on the walls of supercages [9]. Thus it seems probable that the higher activity of NaX zeolite with respect to that of NaY zeolite results from the higher concentration of SH- species in this catalyst. As follows from ‘H NMR data, SH- concentration remains constant if the molecular form of adsorbed H2S is present on the catalyst surface. This may be rationalized assuming that the loss of SH- species in reaction with CH30H is compensated by dissociation of molecular H$ according to the scheme: HBS+ SH- + Hf. The other argument in favor of the significant role played by SH- species in this reaction is the low activity of the H,PO,&iOa catalyst on which SHspecies are not formed [lo]. DME is accumulated in this system due to its low (if any) reactivity towards molecular HzS, but DME readily reacts with SH- species over NaX zeolite, giving first MM and then DMS (Pig. 703’4’)). NMR data do not contradict the literature data on the mechanism of CH,OH+H,S reaction and allow us to elucidate the role of dissociatively adsorbed H,S, which seems to be the form most reactive towards methanol. Summarizing the results of the experiments, it can be concluded that ‘H and 13CMAS NMR permit monitoring of the adsorption forms of reagents,
their interaction in the adsorbed state, the products of reaction (including those not observed by gas chromatography) and obtainment of supplementary data on the mechanism of the H,S+CH,OH reaction.
References 1 M. Ziolek and I. Brezinska, Zeolites, 5 (1985) 245. 2 M. Ziolek, I. Brezinska and H. G. Karge, Acta Phys. Chern. Szeged, 31 (1985) 551. 3 A. V. Mashkina, E. A. Paukshtis, A. N. Yakovleva and G. V. Tiiofeeva, Kin&. Katal., 30 (1989) 1239 (in Russian). 4 J, M. Thomas and J. Klinowski, Adv. Cutal., 33 (1985) 199. 5 V. M. Mastikhin and K. I. Zamaraev, Usp. Khim., 55 (1986) 387. 6 E. G. Derouane, B. Nagy, P. Dejaife, J. H. C. Van Hoff, B. P. Specman, J. C. Vedriie and C. Naccache, J. Cutal., 53 (1978) 40. 7 The Sad&r Standard Spectra: 13C NMR Spectra, Vols. l-120. N lC-24OOOC, Sadtler Research Lab., Philadelphia, 1976-1988. 8 V. M. Mastikhin, I. L. Mudrakovsky, A. V. Nosov and A. V. Mashklna, J. Ch.em. Sot., Far&au Trans. 1, 85 (1989) 2819. 9 H. G. Karge and J. Rask6, J. Co&id Interface Sci., 64 (1978) 522. 10 V. M. Mast&him and 0. B. Lapina, React. Kin.&. CataL L&t., 11 (1979) 353. 11 I. D. Gay, J. Phys. Chem., 84 (1980) 3230. 12 M. W. Anderson and J. KIinowski, J. Am. Chem. Sot., 112 (1990) 10.
of Molecular Catalysis, 66 (1991) 73-83
73
‘H and 13C NMR study of catalytic reaction between CH30H and H2S A. V. NOSOV,V. M. Mastikhin and A. V. Mashkina Institute of Catalysis, Siberian Branch of the Acadmny of Sc&nces, Novosibirsk, 630090 (-US.S.R.] (Received May 30, 1990; revised December 5, 1990)
Abstract The resultsof ‘H and 13C NMR as well as kinetic studies of the reaction between CH30H and HeS over a number of solid catalvsts (SiOe, H3P0&i02, NaX, NaY and H-ZSM-5 zeolites) are presented. The adsorbed methanol forms methoxy groups on H3P0JSi02 and presumably on H-ZSM-5 catalysts while for SiOe, NaX and NaY only physically adsorbed methanol molecules are found on the catalyst surface. The ‘H and 13C NMR spectra in combination with the kinetic data indicate that the adsorbed.CH,OH reacts with SH- species and HzS molecules, the former being more reactive.
Introduction Zeolites as well as oxides are known to catalyze the reaction between CHaOH and H2S, with formation of methylmercaptan (MM) and dimethyl sulphide (DMS). The mechanism of this reaction has not been elucidated. Ziolek et al. [ 1, 21 studied this reaction over HNaY zeolite by means of IR spectroscopy. They obtained some evidence that the reaction proceeds via methoxylation of the zeolite surface, with subsequent reaction between methoxy groups and physically adsorbed H2S. Both MM and DMS are formed at low temperatures; at higher temperatures DMS formation prevails. In [3] Mashkina et al. have analyzed the effect of acid-base properties of a wide range of catalysts on their activity in the reaction of methanol with hydrogen disulphide. They concluded that on catalysts having strong acidic sites the reaction proceeds fust between methoxy groups and dissociatively adsorbed Has, with formation first of MM which transforms into DMS. Recently NMR spectroscopy has become a powerful tool for the study of surface active sites, adsorbed molecules and their reactions over catalyst surface [ 4,5]. Derouane et al. [6] were the first to apply 13CNMR spectroscopy to the study of methanol conversion over ZSM-5 zeolites. The use of magic angle spinning (MAS) results in the high resolution NMR spectra of adsorbed molecules and thus provides more detailed information on the chemical nature of the absorbed species. In this paper we present data on the interaction of CHsOH with H$ over a number of solid catalysts obtained via ‘H and 13C NMR (including 0304-5102/91/$3.50
Q Elsevier Sequoia/Printed in The Netherlands
74
MAS experiments), as well as by kinetic studies of this reaction. The simultaneous measurement of 13C and ‘H NMR spectra have provided us with information on the products of this reaction and allowed us to make some conclusions regarding the reaction mechanism.
Experimental NaX, NaY and H-ZSM-5 zeolites were commercial samples purchased from Gorky Chemical Plant. The framework Si/Al ratios were 2.5, 4.7 and 60 respectively. Commercial SiOz (KSK type, specific area 280 m2 g-‘) was used. The samples were calcined in air at 400 “0 (NaX and NaY) and 500 “C (H-ZSM-5 and Si02) for 2 h. After H3P04 impregnation from aqueous solution, the Si02 samples were dried in air at 110 “C for 5 h, with subsequent calcination in He flow at 400 “C for 2 h. The continuous flow technique, with gas chromatographic analysis of the reaction products, was used for measurements of catalytic activity [3]. Experiments were carried out in the kinetic region on catalyst granules 0.25-0.5 mm diameter at 300-360 “C, P=O.l MPa and H2S:CH30H ratio @I) 0.6 and 1.6 without dilution of reactants at different contact times T, (s). The catalytic activity was characterized by methanol conversion rate W (mm01 g- ’ h-l). The selectivity was determined as a ratio of the product yield to methanol conversion (%). NMR spectra were measured at 300.066 (‘H) and 75.43 MHz (13C) using a Bruker CXP-300 NMR spectrometer at ambient temperature. Prior to the measurements the samples were placed in glass tubes of two types: ‘MAS’ (7 mm o.d. and 12 mm length) for measurements of ‘H and 13C MAS NMR spectra or ‘HR’ (10 mm o.d. and 200 mm length) for measurements of 13C spectra Without sample spinning. Samples were evacuated at 500 ‘C (H3P04/ Si02 - at 400 “C) and P= 10m3 Pa for 6 h. Methanol (30% enriched by 13C isotope) adsorption was carried out from alcohol vapour at room temperature. After CH30H adsorption the samples in ‘HR’ tubes were sealed off from the vacuum line and their 13C spectra were recorded. Then the ‘HR tubes were again sealed to the vacuum line and H2S adsorption was carried out. Samples in ‘MAS’ tubes were sealed off immediately after H2S adsorption and CH30H subsequent co-adsorption; only H2S was adsorbed on some samples. The quantities of adsorbed substances were determined volumetrically. After H2S and CH30H adsorption the samples were heated at temperatures from 100 to 400 “C for 20 min outside the NMR spectrometer, with subsequent measurements of the spectra. The chemical shifts were measured relative to external TMS. The rotation frequency of the sealed samples was 2.7-3 KHz.
75
Results
‘H and 13CNMR spectra CH,OH adswption 13CNMR spectra of CH30H adsorbed on catalysts axe shown in Fig. 1. The spectrum of CH30H/Si02 exhibits a line at 6=52.0 ppm with a width of 300 Hz. (Fig. l(a)). The 13C chemical shift of liquid CH30H is 49.4 ppm [ 71. Small differences in chemical shifts between liquid CH,OH and that
d -L
54.0
C
-JL
0
100
’
’
50
’
6,PPt-l
I
t
’
100
’
’
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’
’
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0 6.r-J
Fig. 1. I% NMR spectra of CHBOH adsorbed on (a) SiOz; (b) H3POJSi02; (c) H-ZSM-5; (d) Nay. Fig. 2. 13CNMR spectra of CH,OH adsorbed on H3P0,/Si02 after Hz3 coadsorption and heating at (a) 100 “C; (b) 200 “C; (c) 250 “C; (d) 300 “C; (e) 350 “C; (f) 400 “C; M= 1.0.
76
adsorbed on SiOz may be due to the differences in the magnetic susceptibilities of the samples. Methanol adsorption on H3P04/Si02 gives rise to two overlapping lines at 6= 51.4 and 60 ppm (Fig. l(b)) belonging to physically adsorbed CHaOH (6= 51.4 ppm) and to some more rigidly bound species, most probably surface methoxy groups. The line at S = 54 ppm is observed in the spectrum of CH,OH/‘H-ZSM-5 (Fig. I(c)). The spectra of methanol adsorbed on NaX and NaY zeolites are very similar. As an example, the spectrum of CH,OH/NaY with the line at 6= 52.4 ppm from physically adsorbed methanol is shown in Pig. l(d). Reaction of CH,OH with Ha (a) H3p041Si02. H$ adsorption on H3P0,JSi02 with preadsorbed CHBOH and subsequent heating at 100 “C does not change the 13C spectrum (Fig. 2(a)). An increase in temperature gives rise to the narrow line at 61 ppm accompanied by a decrease in the intensity of the line at 52 ppm (Pig. 2(b-d)). After heating at 350 “C the latter vanishes and new lines at 6= 31.4 and 9.1 ppm appear. Heating at 400 “C gives rise to a small signal at S- 16 ppm. Chemical shift of the low-field line only slightly deviates from that of liquid dimethyl ether (DME, 6=59.4 ppm [7]) and most probably belongs to physically adsorbed DME. The remaining signals (at 6= 31.4, 16 and 9.1 ppm) may be attributed to the products of thermal decomposition of DME, most probably branched alkanes. (b) H-ZSM-5. After H$ adsorption the line from adsorbed CH30H shifts downfield and broadens compared to the spectrum of CH,OHM-ZSM-5: 6=56.2 and 54.0 ppm, AV,~= 850 and 710 Hz respectively (Pig. 3(a)). Heating the sample at 100 “C gives rise to signals at 29.5 and 6.3 ppm, the low-field line broadens to 940 Hz and becomes slightly asymmetric. The high-field (S=6.3 ppm) signal most probably belongs to MM (liquid MM 6=6.5 ppm [7]). After heating at 160 “C (Pig. 3(a)) the intensity of the signal at low field decreases and the signal at 29.5 ppm increases, while the signal from MM does not change. Heating at 210 “C results in the disappearance of the low-field signal and gives rise to a signal at 6 = 18.7 ppm which we have attributed to DMS (liquid DMS S= 18.5 ppm 171). Heating at 350 “C does not change the spectrum. (c) NuY. H2S adsorption and heating at temperatures from 100 to 200 “C lead to line broadening to 830 Hz (Avln of CH30H adsorbed at room temperature was 200 Hz) (F’ig. 4(a-d)). Signals from DME and MM appear in the spectrum measured after the sample was heated at 250 “C (Fig. 4(e)). Heating at 300 “C gives rise to the line at 6= 18.0 ppm from DMS accompanied by an increase in intensity of the signal from DME (Pig. 4(f)). Heating at higher temperature (400 “C) leads to disappearance of the line from MM (Fig. 4(g)). (d) NuX. Figure 5 shows ‘H and “C NMR MAS spectra obtained for M =0.4. The line widths in 13C MAS spectra are 4 times smaller than those measured without sample spinning. The molecules which do not contain
29.5
8
.
. 100
.
so
.
. 0
. SpIl
100
50
0
G,PPW
Fig. 3. 13C NMR spectra of CH30H adsorbed on H-ZSM-5 after HzS coadsorption (a) and heating at (b) 100 ‘T; (c) 160 “C; (d) 210 “C; (e) 360 “C; M=0.6.
Fig. 4. 13C NMR spectra of CH30H adsorbed on NaY after HzS adsorption (a) and heating at (II) 100 “C; (c) 150 T; (d) 200 “C; (e) 250 “C; Q 300 “C; (g) 400 “C; M=0.4.
carbon atoms, such as H,S (6= 1.0 ppm) and SH- species (S- - 3.6 ppm) [8] along with other compounds ‘visible’ in 13Cspectra, ‘were observed in the ‘H MAS spectra (Fig. 5(a-f)). Heating at 225 “C gives rise to signals from MM, DMS and DME (Fig. 5(c’)). After heating at 250 “C the concentration of DMS on the catalyst surface increases and those of MM and CH30H decrease. The line from MM vanishes after heating at 300 “C and the intensity of the DMS line increases, while the line from DME does not change. After heating at 400 “C (F’ig. 5(f)) the line from CH30H almost disappears, while the intensities of the lines from DMS and DME increase. In the ‘H spectra, the line at 5=4.5 ppm belongs to OH groups of methanol, the line at 2 ppm is due to CH3
78
& 10
so
5
0
25
Od,ppm
-5 6,ppm
Fig. 5. ‘H (a-f) and 13C @‘-f’) NMR MAS spectra of NaX zeolite after (a) H&Sadsorption (5 X 10” molecules g-l); (b, b’) subsequent CHBOH adsorption (1.2 x 102’ g-‘) and heating at (c, c’) 225 “C; (d, d’) 250 “C; (e, e’) 300 “C; (f, f’) 400 “C. Spinning sidebands are marked with asterisks.
protons of DMS and MM, the signal at 3 ppm belongs to CH, protons of methanol and DME. It is well known that on zeolites the dissociative adsorption of HzS occurs with formation of SH- species 191. Recently this result was confirmed by ‘H NMR MAS experiments [8]. According to these data the line at S= 1 .O ppm belongs to the molecular form of adsorbed Has, while the line at S= - 3.6 ppm was attributed to surface SH- species. ‘H spectra show that H2S concentration diminishes as CHaOH conversion increases. It should be mentioned that the concentration of SH- species remains constant while HzS is present on the catalyst surface. The signal from SH- species vanishes only when the signal from adsorbed H2S molecules cannot be seen in the ‘H spectra (Fig. 5(f)). The signal from water formed in the reaction is not observed in the ‘H spectra due to a large line width of about 1 KHz
79
[ lo], which cannot be narrowed upon spinning due to an unfavorable exchange rate with protons of surface OH groups. (e) Nay. Figure 6 shows spectra for M= 1. In contrast to NaX zeolite, the concentration of SH- species is lower due to the smaller Si/Al ratio for this zeolite [9, 81. The signal from methanol OH groups is broadened beyond observation due to the restricted mobility of CH,OH molecules at low surface coverage. Heating of this sample leads to a decrease in the intensity of lines from H2S, CH30H and SH- species, MM being the main product, and quantities of DME and DMS being small. (f) DME +H&3 reaction over Na2i zeolite. DME was obtained in situ by heating adsorbed CHaOH at 400 “C for 30 min (Fig. 7(a’)). All methanol was reacted after this treatment, as can be seen from the 13C spectrum. After contact of the sample with HaS and subsequent heating, the lines from MM and DMS appear in the 13C spectra, accompanied by a corresponding
13
C 5.0
es 61.6 ?7
+;.0
A
,-_
5.0
\
507 I
17.0 ,h. \
507 5.5
I
C’
5p.1
b’ k
10
S
0
-5
6.ppm
6. ‘H (a-e) and 13C @‘-e’) NMR MAS spectra of NaY zeolite after (a) H2S adsorption (2 x 1Olg g-l); (b, b’) subsequent CH30H adsorption (2 x 1020 g-l) and heating at (c, c’) 100 “C; (d, d’) 150 “C; (e, e’) 200 “C.
Fig.
80
3xx3 , , . . *. . a
~
10
0
-10
6,ppm
5’0
25
0 6 pm
Fig. 7. ‘H (a-f) and 13C (a’-f’) NMR MA3 spectra of NaX zeolite after (a, a’) DME (1 X 102’ g-l) and subsequent H&Sadsorption (2 x 102’ g-l) and heating at (b, b’) 150 “C; (c, c’) 250 “C; (d, d’) 300 “C; (e, e’) 350 “C; (f, f’) 400 “C.
decrease in intensity of the DME line. DMS is the final product of this reaction. SH- species appeared only after heating at 150 “C (Fig. 7(b)) and disappeared only after H$ had vanished, as in the case of CH,OH+ H2S reaction. Catalytic experiments In the presence of all catalysts tested in the H@+CH,OH
reaction at different T, MM, DMS and DME were formed. On increasing the contact time (the reaction temperature and M being constant), conversion X increased. Table 1 shows the results of these experiments. (a) H&‘04/Si02. At M= 0.6 and 1.6, r=9-42 s and 360 “C (as well as at lower temperatures), DME is the main product; at larger r MM and DMS were also formed. (b) H-ZSM-5. At M=0.6 and 1.6 and ~=0.1-1.3 s, DMS was the main product; only small quantities of MM were formed and no DME was detected. DME was observed at 260-300 “C and low methanol conversions.
81 TABLE 1 Conversion rates and selectitities of catalysts in H,S + CH,OH reaction at 360 “C, X= 80% Catalyst
SiOz H3P0,/Si02 H-ZSM-5 NaY NaX
S” cm2 g-l)
360 90 500 800 800
M=0.6
M= 1.6
Tb (9)
WC
85 45 0.9 45 7.0
5 2 3 1 214 6 3.5 58 23 56
Selectivity MMd
“Specifk surface area. bContact time. ‘Methanol conversion rate, mm01 g-’ dMethylmercaptan. eDimethyl sulphide. ‘Dimethyl ether.
W
DMS’
DME’
6 10 91 42 44
92 89 0 0 0
$1
34 20 0.3 7.5 2.5
2 6 250 12 37
Selectivity MM
DMS
DME
25 3 19 72 83
6 30 81 28 17
69 87 0 0 0
h-l.
(c) NaY. At M=0.6 and 1.6 and T= 3-30 s, methanol was converted to MM and DMS; no DME was observed. The selectivity towards MM decreased at increased conversion. (d) NaX. The reaction rates on this zeolite were 7-9 times larger compared with those on NaY, selectivity being almost the same.
Discussion Figure 1 shows that methanol bonding with the surface depends upon the catalyst type. On SiOa, NaX and NaY zeolites, physically adsorbed methanol predominates at coverages used in the experiment (0=0.1-l monolayer). This conclusion follows from the proximity of 13C chemical shifts of liquid, gaseous and adsorbed CH,OH, and from observation of the line from the OH group of methanol adsorbed on NaX zeolite in the ‘H spectra. After CH30H adsorption on H3P04/Si02 two lines were observed, one of them only slightly different from the line of CH30H adsorbed on SiOa. Another line with larger line width and with a chemical shift of N 60 ppm most probably. belongs to Me- O-P groups. The signal of CH,OH adsorbed on H-ZSM-5 zeolite was shifted and broadened compared to methanol adsorbed on SiOz. This may be due to fast exchange between physically adsorbed CH30H and surface methoxy groups. Formation of methoxy groups has been studied by IR spectroscopy on HNaY zeolites [l] as well as by i3C NMR on MgO [ 111. In the latter case, the value of the isotropic chemical shift of methoxy groups was reported to be 60 ppm. The present data indicate that methoxylation occurs on H3P04/
82
SiOz and H-ZSM-5 zeolite. One might expect, however, that the surface of NaY and NaX zeolites, which have small quantities of acidic OH groups, can be also methoxylated to some extent. The formation of a complex between adsorbed CH,OH and H2S seems to take place on NaY zeolite. This follows from the broadening of the methanol 13C signal. No detectable change in chemical shift or appearance of new lines was observed. The line broadening may be explained by decreased mobility of methanol due to formation of its complex with H2S. The bonding energy between HaS and CH,OH molecules in such a complex seems to be rather small because the chemical shift of methanol does not change. The comparison of NMR and gas chromatographic results should be performed with care, because there can be a considerable difference in the data obtained in gas flow and static NMR experiments, as was pointed out recently in [ 121. Keeping this in mind one can see, however, that these two sets of data are in rather good qualitative agreement. Data on reaction over H-ZSM-5 zeolite is the only exception: the product with 5=29.1 ppm is observed in 13C spectra but it is absent in catalytic experiments. This unambiguously indicates that the reaction product having the line with 6 = 29.1 ppm does not desorb from the zeolite surface. Traces of this compound appear also in the reaction over NaY zeolite (Pig. 5(e-f)). Its chemical composition remains unclear, but it should contain sulfur, because no signal with this chemical shift was observed during methanol thermal decomposition on this catalyst. The formation of this product may probably explain the deactivation of this catalyst observed in catalytic experiments. According to literature data, the CH30H + H&l reaction proceeds between methoxy groups and H2S molecules and/or SH- species [l-3]. ‘H NMR is a suitable tool to monitor both HaS and SH- species in this reaction. Concentration of SH- . - *Na+ species on NaX zeolite is greater than that on NaY zeolite, due to a larger concentration of Na+ cations is S(II1) sites on the walls of supercages [9]. Thus it seems probable that the higher activity of NaX zeolite with respect to that of NaY zeolite results from the higher concentration of SH- species in this catalyst. As follows from ‘H NMR data, SH- concentration remains constant if the molecular form of adsorbed H2S is present on the catalyst surface. This may be rationalized assuming that the loss of SH- species in reaction with CH30H is compensated by dissociation of molecular H$ according to the scheme: HBS+ SH- + Hf. The other argument in favor of the significant role played by SH- species in this reaction is the low activity of the H,PO,&iOa catalyst on which SHspecies are not formed [lo]. DME is accumulated in this system due to its low (if any) reactivity towards molecular HzS, but DME readily reacts with SH- species over NaX zeolite, giving first MM and then DMS (Pig. 703’4’)). NMR data do not contradict the literature data on the mechanism of CH,OH+H,S reaction and allow us to elucidate the role of dissociatively adsorbed H,S, which seems to be the form most reactive towards methanol. Summarizing the results of the experiments, it can be concluded that ‘H and 13CMAS NMR permit monitoring of the adsorption forms of reagents,
their interaction in the adsorbed state, the products of reaction (including those not observed by gas chromatography) and obtainment of supplementary data on the mechanism of the H,S+CH,OH reaction.
References 1 M. Ziolek and I. Brezinska, Zeolites, 5 (1985) 245. 2 M. Ziolek, I. Brezinska and H. G. Karge, Acta Phys. Chern. Szeged, 31 (1985) 551. 3 A. V. Mashkina, E. A. Paukshtis, A. N. Yakovleva and G. V. Tiiofeeva, Kin&. Katal., 30 (1989) 1239 (in Russian). 4 J, M. Thomas and J. Klinowski, Adv. Cutal., 33 (1985) 199. 5 V. M. Mastikhin and K. I. Zamaraev, Usp. Khim., 55 (1986) 387. 6 E. G. Derouane, B. Nagy, P. Dejaife, J. H. C. Van Hoff, B. P. Specman, J. C. Vedriie and C. Naccache, J. Cutal., 53 (1978) 40. 7 The Sad&r Standard Spectra: 13C NMR Spectra, Vols. l-120. N lC-24OOOC, Sadtler Research Lab., Philadelphia, 1976-1988. 8 V. M. Mastikhin, I. L. Mudrakovsky, A. V. Nosov and A. V. Mashklna, J. Ch.em. Sot., Far&au Trans. 1, 85 (1989) 2819. 9 H. G. Karge and J. Rask6, J. Co&id Interface Sci., 64 (1978) 522. 10 V. M. Mast&him and 0. B. Lapina, React. Kin.&. CataL L&t., 11 (1979) 353. 11 I. D. Gay, J. Phys. Chem., 84 (1980) 3230. 12 M. W. Anderson and J. KIinowski, J. Am. Chem. Sot., 112 (1990) 10.